1. Field of the Invention
The present invention relates to an air flow control device of an engine.
2. Description of the Related Art
Most of the unburned hydrocarbons emitted from an engine during its operation from when the engine is started to when the engine is stopped are emitted at the start of the engine. Accordingly, in order to reduce the amount of emission of the unburned hydrocarbons during engine operation, it becomes necessary to reduce the amount of emission of the unburned hydrocarbons at the start of the engine. Usually, in an engine, however, a large volume portion such as a surge tank is provided downstream of the throttle valve. As a result, at the time of cranking, the area downstream of the throttle valve is maintained at substantially atmospheric pressure. In other words, at the time of cranking of the engine, a large amount of air is fed into the engine cylinders like at the time of a high load operation. Accordingly, at the time of cranking of the engine, a large amount of fuel corresponding to this large amount of air is fed, so a large amount of unburned hydrocarbons are generated. In this case, if the amount of intake air fed into the engine cylinders can be reduced at the time of cranking of the engine, the amount of feeding of fuel can be reduced as well and thus the amount of emission of the unburned hydrocarbons can be reduced.
There is a known engine in which the amount of intake air fed into the engine cylinders is reduced at the time of cranking of the engine (refer to Japanese Unexamined Utility Model Publication (Kokai) No. 1-119874). In this engine, an air flow control valve is arranged in the intake passage downstream of the throttle valve. The air flow control valve is driven by a vacuum operated diaphragm type drive device. At the time of cranking of the engine, the air flow control valve is held in the closed state by the vacuum operated diaphragm type drive device. When the engine speed starts to rise and it exceeds a predetermined engine speed, a valve open signal for the air flow control valve is given to the vacuum operated diaphragm type drive device, and the air flow control valve is opened. In this engine, since the air flow control valve is held in the closed state at the time of cranking of the engine, the amount of intake air fed into the engine cylinder is reduced.
In such a vacuum operated diaphragm type drive device, however, there is a operating delay, that is, there is a constant time delay from when the valve open signal for the air flow control valve is given to when the air flow control valve actually opens. As a result, during the delay period in which the air flow control valve is held in the closed state, the amount of intake air fed into the engine cylinder is suppressed, and accordingly the engine speed does not rise so much. The engine speed first rises up to the target engine speed only when the air flow control valve opens. Namely, there is a problem in that a good feeling of engine start cannot be obtained since the engine speed rises in stages at the start of the engine.
Further, since the target engine speed at the time of start of the engine is set near the lower limit engine speed with which a good combustion is obtained, the combustion becomes unstable during a period in which the engine speed does not reach the target engine speed due to the valve opening delay of the air flow control valve. As a result, there arises a problem in that not only is the amount of emission of the unburned hydrocarbons increased, but also an extremely large amount of unburned hydrocarbons is emitted if misfiring occurs in any cylinder.